Sleep disruption is common during spaceflight. A survey of 58 crew members from 9 shuttle missions revealed that most suffered from sleep disruption and were unable to sleep more than six hours per day of flight as compared to 7.9 hours per day on the ground. Nineteen percent of crew members on single shift missions and 50 percent of the crew members in dual shift operations reported sleeping pill usage (benzodiazepines) during their missions. Although benzodiazepines are effective as hypnotics, their adverse next-day side effects include sedation, performance decrements, amnesia and distortions in the sleep EEG. Our preliminary data suggest that the pineal hormone melatonin, which has been reported to modulate the output of the human circadian pacemaker, may also have the acute hypnotic properties needed for treating the sleep disruption of spaceflight, without producing the adverse side effects associated with benzodiazepines. The applicants hypothesize that pre-sleep administration of melatonin will result in decreased sleep latency, reduced nocturnal sleep disruption, improved sleep efficiency and enhanced next-day alertness and cognitive performance both in ground-based simulations and during the Neurolab mission. Double-blind placebo-controlled trials are proposed in which: (1) the effectiveness of melatonin as a hypnotic is assessed independent of its effects on the phase of the endogenous circadian pacemaker in ground- based studies, using a powerful experimental model of the dyssomnia of spaceflight; and (2) the effectiveness of melatonin as a hypnotic is assessed during the Neurolab mission. In both experiments the effects of melatonin on sleep stages and spectral composition of the EEG during sleep will be determined as well as its effects on daytime alertness and performance. The proposed research could have significant implications for both sleep disorders medicine and aerospace medicine. The results of the proposed research could lead to the development of a new treatment regimen for sleep disturbances of various etiologies during spaceflight, which could enable crew members to avoid the decrements in alertness and performance associated with sleep deprivation. This work could therefore have a profound impact on the health, productivity and safety not only of astronauts during space flight, but also of other groups with a high prevalence of insomnia, such as shift workers and the elderly.